Re: [Lsf-pc] [LSF/MM TOPIC] Persistent Memory

[Bob Liu <bob.liu@oracle.com>]

On 01/08/2014 11:42 PM, Mel Gorman wrote:
> On Fri, Dec 20, 2013 at 10:05:02AM -0700, Matthew Wilcox wrote:
>>
>> I should like to discuss the current situation with Linux support for
>> persistent memory.  While I expect the current discussion to be long
>> over by March, I am certain that there will be topics around persistent
>> memory that have not been settled at that point.
>>
>> I believe this will mostly be of crossover interest between filesystem
>> and MM people, and of lesser interest to storage people (since we're
>> basically avoiding their code).
>>
>> Subtopics might include
>>  - Using persistent memory for FS metadata
>>    (The XIP code provides persistent memory to userspace.  The filesystem
>>     still uses BIOs to fetch its metadata)
>>  - Supporting PMD/PGD mappings for userspace
>>    (Not only does the filesystem have to avoid fragmentation to make this
>>     happen, the VM code has to permit these giant mappings)
> 
> The filesystem would also have to correctly align the data on disk. All
> this implies that the underlying device is byte-addressible, similar access
> speeds to RAM and directly accessible from userspace without the kernel
> being involved. Without those conditions, I find it hard to believe that
> TLB pressure dominates access cost. Then again I have no experience with
> the devices or their intended use case so would not mind an education.
> 
> However, if you really wanted the device to be accessible like this then
> the shortest solutions (and I want to punch myself for even suggesting
> this) is to extend hugetlbfs to directly access these devices. It's
> almost certainly a bad direction to take though, there would need to be a
> good justification for it. Anything in this direction is pushing usage of
> persistent devices to userspace and the kernel just provides an interface,
> maybe that is desirable maybe not.
> 
>>  - Persistent page cache
>>    (Another way to take advantage of persstent memory would be to place it
>>     in the page cache.  But we don't have struct pages for it!  What to do?)
> 

I think one potential way is to use persistent memory as a second-level
clean page cache through the cleancache API.

-- 
Regards,
-Bob

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